Screw and Bolt Manufacturing Materials: A Complete Guide

Steel in Fasteners

Stainless steel fasteners are among the most widespread and versatile available in the market because, given the mechanical and chemical characteristics of steel, this turns out to be one of the most requested and used materials.

Steel is an alloy composed of two main elements: iron and carbon. Iron (Fe) is a metallic element while carbon (C) is the main bonding element within alloys.

The percentage of carbon present in steel varies but generally fluctuates between 0.002% and 2.1% (with a percentage higher than this maximum, it is referred to as cast iron). Along with these two elements, others such as manganese, phosphorus, sulfur, silicon, copper, etc., can also be found. Depending on the percentage with which these elements can be present, the characteristics of the steel change, and consequently, different hardnesses, corrosion resistances, heat resistances, and tensile strengths will be encountered.

Steel is classified into 4 main categories:

• Carbon steel;
• Alloy steel;
• Tool steel;
• Stainless steel.

This last type of metal, also known as stainless steel, is a material highly resistant to oxidation thanks to the presence of chromium, which ranges between 10% and 20%. Most fastening products are produced in stainless steel A2 and A4, materials used in many sectors thanks to their resistance to corrosion, good polishing, and a high hygiene coefficient. The difference between stainless steel A2 and A4 is not perceptible to the naked eye, but however, some specifics make them different.

Example of stainless steel fasteners

Among the characteristics that make a difference in the classification of steel is resistance, i.e., the ability to resist an impressed force without undergoing deformations. Steels can be high strength or low strength, thus having different plasticity. In this regard, it is important to know that there is a reference standard, namely UNI EN ISO 898-1, which defines the criteria for the resistance classes of screws and bolts.

Knowing all the characteristics of steel and the various declinations of this material means choosing correctly based on the final application. For example, while stainless steel is to be used in conditions where a high anticorrosive capacity is required, carbon steel is more easily adapted to the production of knives, high-tension wires, and car bodies because it is easier to work with.

Brass: Versatility and Applications

Brass is among the materials most used in the production of fasteners. Indeed, this alloy of copper finds multiple applications thanks to its good mechanical characteristics, high resistance to oxidation, and also for purely aesthetic reasons.

In general, brass is a compound of copper and zinc with variable percentages of zinc based on usage needs. This material is classified into:

• Ternary brasses, made only of copper and zinc;
  
  
• Binary brasses, where other chemical elements are also present;
  
  
• Quaternary brasses, composed of the presence in the alloy of a fourth chemical element.

A further classification of brass is related to the temperatures and compositions of the alloys.

In detail, these can crystallize into three different solid solutions:

• Brass alloys with zinc percentages greater than 45%. These are elements with very high brittleness values and, for this reason, do not find practical applications;
  
  
• Brass alloys with zinc percentages between 36% and 45%. These are alloys easily workable at high temperatures;
  
  
• Brass alloys with zinc percentages lower than 30-35%. These are elements endowed with high cold plasticity and brittleness at high temperatures.

Among the most used brass fastening elements are washers, which can be applied for small or large thicknesses. In the first case, they are obtained by stamping, while in the second through mechanical processing.

Brass fasteners

Cast Iron: When and Why to Use It

Cast iron is an Iron-Carbon alloy in which the Carbon (C) content exceeds 2.06%. The carbon present within the cast iron solidifies in the form of graphite, which, in turn, can appear in lamellar form or in spheroidal form. These two forms of Carbon solidification determine the two main families of cast iron:

• Lamellar Cast Iron
  
  
• Spheroidal Cast Iron

From these two families, it is possible to classify the alloy into further subgroups, depending on the ferrous matrix of the alloy itself. We then speak of:

• Ferritic Cast Iron
  
  
• Pearlitic Cast Iron
  
  
  

Production of cast iron In general, cast iron is a very hard material but, at the same time, presents low resistance to traction and bending. For this reason, it cannot be subjected to plastic work since it is not malleable either at high or low temperatures. Nevertheless, it is a material often considered for its resistance to compression and corrosion, characteristics that make it suitable for specific work.

As for the production of fasteners, cast iron does not find application. At the same time, however, thanks to its hardness and resistance to high stresses and compressions, it is used for the production of other fastening elements, such as support plates and spacers.

Copper: Conductivity and Resistance

Among the most appreciated materials in the production of fasteners is copper, a substance able to guarantee high resistance to corrosion.

Copper screws hardly show rust or corrode and can therefore be applied even in very humid environments. Moreover, given its excellent electrical conductivity, copper is used for the production of special screws or washers.

Just as with brass products, copper screws and rivets of small diameter are produced by stamping, while flat washers by cutting.

Among the sectors where this material finds greater use are electromechanics and tinsmithing.
In these two areas, except for special alloys, the copper used has a minimum purity greater than 99.90% and can belong to two different types that differ in the presence of other elements:

• Cu-ETP, is copper characterized by the presence of Oxygen and the absence of Phosphorus, a combination that gives it high electrical conductivity;
  
  
• Cu-DHP, is copper completely devoid of Oxygen but has a Phosphorus value, between 0.015% and 0.04%, which reduces the material's conductivity but improves its cold deformability.
  
  
  

Aluminum: Lightness and Resistance

When talking about aluminum fasteners, we refer to elements characterized by low mechanical resistance and a notable ability to resist corrosion. Moreover, they are components that present excellent malleability and thus can be subjected to more or less strong stresses without suffering structural damage.

In general, this material is used for the production of washers and rivets, chosen for coatings but also for structural uses. Conversely, it is rare for aluminum to be used for the production of screws.
In fact, aluminum screws are rarely used and only in specific situations where it is necessary to avoid galvanic corrosion (corrosion that is triggered by contact with more electropositive materials, such as, for example, steel).

Aluminum is not a metal found in nature but is obtained from Bauxite and, generally, is used in the form of an alloy with other elements that influence its mechanical characteristics.

Titanium: High Resistance and Lightness

Like aluminum, titanium is not a material found in nature in its pure state but is contained in minerals such as Rutile and Ilmenite.
This light metal, white in color, shiny, and characterized by good corrosion resistance.

Titanium screws are used in specific sectors precisely for the particular characteristics of this element. Indeed, since titanium is an alloy as resistant as steel but 40% lighter and is heavier than aluminum (about 60% more) but twice as resistant, it is employed in sectors such as automotive, nautical, chemical, and petrochemical industries, areas that require the use of specific fastening elements.

In general, titanium is used in alloy with aluminum to produce light and resistant components. In this regard, three types of titanium alloys can be distinguished:

• Alpha alloys, with higher mechanical resistance and oxidation;
  
  
• Alpha-beta alloys, with less deformability compared to the first but high mechanical resistance and technical malleability. For this reason, they are the most used titanium alloys in the industry (about 70%);
  
  
• Beta alloys, less dense but more malleable, are resistant and light and are mainly used in the aerospace and mechanical industries.

Plastic: Innovation in Fasteners

Plastic fasteners are a set of products that offer several advantages in their use. Plastic fastening elements indeed guarantee:

• A high degree of resistance to vibrations;
  
  
• Absence of electrical conductivity;
  
  
• Protection of metal surfaces from corrosion risk;
  
  
• Reliability even after frequent unscrewing.

There are numerous fields where such fasteners find application, such as in the mechanical sector. Precisely because plastic screws are products with a lower weight compared to other materials but with high ductility and resistance, they find a wide and innovative field of application.
At the same time, it should be noted that such fasteners are characterized by poor resistance to high temperatures and, therefore, cannot be applied in situations where there is a high risk of overheating.

In general, the final characteristics of these elements can differ from each other and depend on the pressure, processing time, and temperature at which the polyamides used for the creation of plastic fasteners are subjected.

Plastic screws and nuts

Nomenclature and Types of Screws and Bolts

The material in which a screw is made is certainly a characteristic through which the fastening element can be classified, but it is not enough. In order to make suitable choices, it is important to know, in general, how many types of screws exist and the various degrees of resistance they can have.

The starting point is knowing the two main parts that make up the screw:

• The head, which can be cylindrical or TC, round, flat, hexagonal, Allen, etc...;
  
  
• The threading, i.e., the spiral that extends the length of the screw and allows its insertion into the chosen material.

Based on these two specifics, the screw can be chosen for material and type of intervention to ensure a result that lasts over time and stable fastening.

Another decisive factor in choosing the right product is the size of screws and bolts. Generally, the greater the length and diameter of the screws, the greater the anchoring power; hence, every product must be carefully selected depending on the final use.

Resistance and Durability: Which to Choose?

In addition to the substance in which the screws are produced, it is essential to carefully evaluate the material in which the elements will be inserted, in order to select the necessary resistance and durability of the product. The components can then be divided into:

• Wood screws, partially or totally threaded elements with a pointed end that can be screwed in without the risk of damaging the material; generally found in galvanized, bronzed, and stainless steel;
  
  
• Drywall screws, usually black and totally threaded;
  
  
• Metal screws, also known as mechanical screws, are available in the market in various shapes, lengths, pitches, and heads; the choice of one mechanical screw over another depends on where they will be applied and the material they are composed of: from the latter will depend on the resistance to temperature and torsion.

In addition to these main categories, we can cite other specific types of screws based on use, such as:

• Sheet metal screws;
  
  
• Frame screws;
  
  
• Concrete screws;
  
  
• Self-drilling screws for thin metals;
  
  
• Self-drilling screws for wood, iron, or sheet metal.

The choice of the product will therefore depend on the evaluation of the individual elements and specific applications. Thus, to consider:

• The material of which the screw is composed;
  
  
• The weight to be supported;
  
  
• The screw's resistance.
  
  

Production Processes and Realization

To have a complete picture of the world of fasteners, it is functional to also know the basic technological treatments to which it is subjected, i.e., how screws and bolts are produced. The three main procedures are distinguished in:

• Chip removal, a mechanism that starts from the use of a hard and sharp tool, which exports a full piece of material to obtain the desired shape. This process is used with materials able to flake off easily, so hard and porous ones, which could make processing difficult, are to be avoided;
  
  
• Cold plastic deformation, is a process that occurs at high speed because it follows now standardized steps. It starts from a bar, or rod, which is accurately drawn to allow optimal filling of the mold. It is a technique that, by using a very massive fixed frame, allows the processing of hard materials like steel;
  
  
• Hot plastic deformation, a technique similar to the cold one but used for the realization of products of larger sizes and diameters. For this reason, the machine is fed by a bar, whose end is heated by induction heat before being cut. This type of processing is applicable to materials such as steel, aluminum, copper, and brass.

Each of these processes is therefore dedicated to the specific production of products, but not always can such a schematic division be followed.
In fact, for some productions, mixed treatments are necessary, which result in specific criteria of robustness and corrosion resistance.

Tips for Choosing the Right Material

A screw or bolt must be primarily suitable in relation to the final needs.
That's why, in order to choose the right material and therefore the suitable product, one must take into consideration:

• The type of screw head, to achieve greater tightening;
  
  
• The environment/application field, to calculate a stronger or weaker resistance to corrosion;
  
  
• The treatment the screw is subjected to, as this factor is also decisive to avoid corrosion and thus ensure greater durability over time;
  
  
• Resistance, given that screws and bolts will guarantee a better result if they have the same resistance and the same pitch;
  
  
• The material in which the component is produced and the one in which it will be inserted.

Only through careful evaluation of the fasteners will it be possible to choose the right element for one's operations.

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